Metal coating compositions prepared from aqueous dispersions of vinyl chloride/alkene copolymers

ABSTRACT

Metal surfaces are coated with an aqueous dispersion of a water dispersible vinyl chloride/alkene copolymer and a mixture of heat-curable, thermosetting aminoplast resins.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention is related to the coating of metal articles, and moreparticularly to coating metal articles with aqueous dispersions of vinylchloride/alkene polymers.

2. The Prior Art

In the manufacture of metal containers, a protective synthetic resincoating is applied to the interior of the container. The syntheticresins which are employed for coating the interior of the metalcontainer are generally resinous materials which are applied in the formof a solution or dispersion in a volatile organic solvent. Thewet-coated surface is passed through an oven which hot air is circulatedto evaporate the solvent and to cure the coating material to therequired hardness. The removal of the organic solvent creates an airpollution problem which many present day communities will not tolerate.

Among the various methods which have been proposed to avoid the use oforganic solvents in preparing synthetic resin coatings for metalsurfaces is to formulate the coating as an aqueous dispersion.

Aqueous dispersions of vinyl chloride/alkene copolymers have beenproposed by the art as coating materials. These copolymers have notfound wide commercial application in the food container industry becauseof the extremely high standards of coating integrity required for foodcontainers, especially in those containers used for packaging carbonatedbeverages such as beer and soft drinks.

In the packaging of carbonated beverages, the interior coating film issubjected to an acidic liquid under high pressure. Under such demandingconditions, it is extremely critical that any coating applied to theinterior of the container be as inert as possible to the packagedcontents as well as processing conditions. To date aqueous dispersionsof coating materials based on vinyl chloride/alkene copolymers have notwholly met the requirements of the carbonated beverage industry.

SUMMARY OF THE INVENTION

In accordance with the present invention, there is provided dispersed inan aqueous medium, a coating composition for application to metalsurfaces to provide an adherent, substantially inert coating film whichis comprised of a vinyl chloride/alkene copolymer and a heat curable,thermosetting aminoplast resin mixture comprised of a methylolatedethylene-urea resin and a alkylated triazine/formaldehyde condensate.

As will hereinafter be illustrated, the aqueous coating compositions ofthe present invention when applied to metal surfaces such as aluminumand steel, provide a coating film of sufficient adhesion and inertnessthat the coating compositions meet the exacting specifications requiredfor containers used by the carbonated beverage industry.

PREFERRED EMBODIMENTS

The vinyl chloride/alkene copolymers used in the practice of the presentinvention are copolymers containing about 70 to 99 percent by weightvinyl chloride and about 1 to 30 percent by weight of an alkene having 2to 4 carbon atoms, namely ethylene, propylene, n-butene or isobutene.Vinyl chloride/ethylene copolymers are preferred in the practice of thepresent invention.

The vinyl chloride/alkene copolymer is prepared by methods known to theart. Aqueous latices of vinyl chloride/alkene copolymers areconventionally prepared by polymerizing the monomers by suspension oremulsion polymerization in the presence of a redox catalyst system andan emulsifying agent in water at elevated pressures. The copolymer latexobtained generally contains 40 to 70 percent solids.

To prepare aqueous coating compositions suitable for coating metalsurfaces, a mixture of the thermosetting methylolated ethyleneurea resinand alkylated triazine/aldehyde resin is incorporated in the vinylchloride/alkene latex.

Methylolated ethyleneurea resins are known to the art and are preparedby reacting ethyleneurea or a derivative thereof with formaldehyde,acetaldehyde, acrolein, benzaldehyde, furfural or other equivalentaldehyde. Methylolated ethyleneurea resins are available commercially.For example, Rhonite R-1, a product of Rohm and Haas is N,N' dimethyolethyleneurea, a condensate of ethyleneurea and formaldehyde. Anotherexample of a methylolated ethyleneurea resin is dihydroxydimethylolethyleneurea.

Water dispersible alkylated triazine aldehyde condensates are also knownto the art and are prepared by the condensation of an amino-triazinesuch as melamine, benzoquanamine and acetoquanamine with an aldehydesuch as formaldehyde, acetaldehyde, benzaldehyde and the like in thepresence of a monohydroxyl alcohol such as methanol and ethanol. Waterdispersible alkylated triazine aldehyde condensates are availablecommercially. For example, Cymel 301 available from American Cyanamid ishexamethoxymethyl melamine, an alkylated triazine aldehyde condensateprepared by the reaction of melamine and formaldehyde in the presence ofmethanol.

It is an essential and critical feature of the present invention thatthe vinyl chloride/alkene copolymer coating compositions be preparedfrom a mixture of the methylolated ethyleneurea resin and the alkylatedtriazine aldehyde condensate. As will hereinafter be illustrated, ifeither one of these aminoplast resins is omitted from the vinylchloride/alkene copolymer aqueous dispersions, the resultant coatingcompositions are unsuitable for metal coating applications.

The aqueous dispersions used in the practice of the present inventionare prepared by dispersing the aminoplast resins in the aqueous latexcontaining the vinyl chloride alkene copolymer and the emulsifying agentused in the preparation of the latex. Such emulsifying agents may beanionic, cationic or non-ionic. Examples of anionic agents are thesodium salts of sulphated fatty acids such as sodium lauryl sulphate.Examples of cationic agents are quaternary ammonium compounds such asstearamidopropyl dimethyl β -hydroxyethyl ammonium nitrate. Examples ofnonionic agents are block copolymers of propylene oxide and ethyleneoxide.

The solids content of coating compositions of the present invention arecomprised of 70 to 95% by weight and preferably 80 to 90% by weight ofthe vinyl chloride/alkene copolymer, 5 to 20% by weight and preferably 8to 15% by weight of the methylolated ethyleneurea resin and about 1 toabout 10% by weight and preferably 2 to 5% by weight of the alkylatedtriazine/aldehyde condensate.

The coating compositions of the present invention can be satisfactorilyapplied at a solids content ranging from about 40 to about 70% byweight, based on the total weight of the aqueous dispersion. Generally,solids content of 45 to 60% by weight is preferred.

The aqueous coating compositions of the present invention can besatisfactorily applied by any of the conventional methods employed bythe coating industry. However, for coating the inner walls of metalcontainers, spray coating is preferred method, as the desired coatingweight is easily and conveniently applied in a single coat. For generalcoating purposes, roll, dipping and flow coating are also useful methodsof application.

After applying the aqueous coating dispersion, the applied coating iscured by heating the coated metal substrate at a temperature of about300°F to about 450°F for a period of about 2 minutes to 10 minutes.

The preferred coating weight for coating metal container bodies with anadequately protective coating is in the range of 1.0 to 10.0 milligramsof resin coating per square inch of exposed metal surface. To obtainthis coating weight, the aqueous resin dispersion is applied at acoating thickness of about 0.1 to 1.0 mils.

The present invention is illustrated but not limited, by the followingexample. Unless otherwise specified all percentages are by weight.

EXAMPLE

To a latex containing dispersed therein 3 percent sodium lauryl sulfate,44.64 percent of a vinyl chloride/ethylene (VCl/E) copolymer having amolecular weight of 40,000 and containing about 80 percent vinylchloride and 20 percent ethylene dispersed in 49.12 percent water wasadded 4.46 percent dimethylol ethyleneurea (DMEU) (Rhonite 1) and 1.78percent hexamethoxymethyl melamine (HMMM) (Cymel 301) to prepare adispersion of the aminoplast resins and the vinyl chloride copolymer.

The aqueous coating dispersion was spray coated on the interior of canbodies fabricated from tin-free steel of the type and gradecorresponding to that conventionally employed in the fabrication ofcarbonated beverage containers. The coated cans were baked for 3.5minutes at an oven temperature of 350°F. The baked coatings had athickness of 0.4 mils.

The quality of the coatings cured on the metal substrate was evaluatedfor adhesion and water resistance. The test results are recorded in theTable below.

For purposes of contrast, as a control test, steel container bodieswhich had been coated in a similar manner but either Rhonite R-1 orCymel 301 had not been added to the coating dispersion were alsoevaluated. The results of these control tests (designated by the symbol"C") are also recorded in the Table.

DRY ADHESION

Adhesion of the baked coating film was determined by scoring the coatingfilm with a sharp metal point in the shape of an "X" and then pressing apiece of adhesive cellophane tape against the "X" score and the tapepulled to determine whether the coating film could be lifted from thesteel surface. Adhesion was rated Poor (P), if substantially all of thecoating film was removed, Good (G), if some portions of the film wereremoved, Very Good (VG), if a very small amount of the film was removed,and Excellent (E), if no coating was removed. In order to be acceptablefor commercial use, the adhesion of the coating must have at least a VGrating.

WATER RESISTANCE

The resistance of the baked coating to pasteurization conditions wasdetermined by placing the coated containers for 30 minutes in agitatedwater baths heated at 155°F and 212°F (boiling water) and thendetermining whether any blushing, blistering or discoloration of thecoating occurred. Water resistance was rated Poor (P), if there wassubstantial blistering and discoloration of the coating, Good (G), ifthere were some blistering and discoloration of the coating, Very Good(VG), if there was very little blistering and discoloration of thecoating and Excellent (E), if there was no change in appearance of thecoating.

WET ADHESION

The procedure used in the Dry Adhesion test was used to determine thewet adhesion of the coating after the coated container was removed fromimmersion in the boiling water bath.

                                      TABLE                                       __________________________________________________________________________    COMPOSITION OF                                                                COATING (Percent)    WATER RESISTANCE                                         Test                 Dry            Wet                                       No.                                                                              VCl/E                                                                              DMEU                                                                              HMMM H.sub.2 O                                                                         Adhesion                                                                           155°F                                                                       212°F                                                                       Adhesion                                  __________________________________________________________________________    1  44.64                                                                              4.46                                                                              1.78 49.12                                                                             E    E    E    E                                         C.sub.1                                                                          50   --  --   50.0                                                                              E    P    P    P                                         C.sub.2                                                                          45.45                                                                              4.55                                                                              --   50.0                                                                              E    P    P    P                                         C.sub.3                                                                          40.32                                                                              9.68                                                                              --   50.0                                                                              E    P    P    VG                                        C.sub.4                                                                          35.71                                                                              14.29                                                                             --   50.0                                                                              E    P    P    VG                                        C.sub.5                                                                          49.02                                                                              --  1.96 49.02                                                                             E    P    P    G                                         C.sub.6                                                                          44.62                                                                              --  10.75                                                                              44.63                                                                             E    P    P    VG                                        C.sub.7                                                                          41.66                                                                              --  16.67                                                                              41.67                                                                             E    P    P    VG                                        C.sub.8                                                                          --   83.33                                                                             16.67                                                                              0   P    P    P    P                                         C.sub.9                                                                          --   50.00                                                                             50.00                                                                              0   P    P    P    P                                         __________________________________________________________________________

By referring to the Table, it is immediately apparent that coatingcompositions comprised of a mixture of a vinyl chloride/ethylenecopolymer, a methylolated ethyleneurea resin and an alkylated triazinealdehyde condensate exhibit superior adhesion and water resistance whencompared to coating compositions in which one of these components is notpresent in the coating formulation.

What is claimed is:
 1. A dispersion composition comprised of a vehicledispersed in water, the vehicle comprising about 40 to about 70% byweight of the dispersion and comprised ofa. about 70 to 95 percent byweight of a vinyl chloride/alkene copolymer comprised of about 70 to 99percent by weight vinyl chloride and about 1 to 30 percent by weight ofan alkene having 2 to 4 carbon atoms, b. about 5 to about 20 percent byweight of a methylolated ethyleneurea resin prepared from a reactionmixture comprised of an ethyleneurea and an aldehyde, and c. about 1 toabout 10 percent by weight of an alkylated triazine aldehyde condensateprepared by the reaction of an aminotriazine and an aldehyde in thepresence of a monohydroxyl alcohol.
 2. The composition of claim 1wherein the alkene is ethylene.
 3. The composition of claim 1 whereinthe methylolated ethyleneurea resin is the reaction product ofethyleneurea and formaldehyde.
 4. The composition of claim 1 wherein thealkylated triazine aldehyde is the reaction product of melamine,formaldehyde and methanol.